This invention relates to a liquid pump for moving the mobile phase in high-performance liquid chromatography (HPLC).
In HPLC, a pump serves to cause the mobile phase solution to be transported to the column after its deaeration by continuously applying pressure to it with a plunger. FIG. 3 shows an example of prior art pump of this kind, comprising a pump head 21, a seal holder 22, a head holder 25, a plunger assembly 27, a return spring 28 and a housing 26. The pump head 21 has an internal flow route 21a therethrough with unidirectional valves at its inlet and outlet sides. The plunger assembly 27 includes a plunger rod 27a which undergoes a reciprocating motion inside the pump head 21, the seal holder 22 and the head holder 25, as well as a spring seat 27b. The return spring 28 is supported by this spring seat 27b and is biased from behind by a cross head 29.
Pumps, used in HPLC, must be disassembled periodically for inspections because the plunger assembly must be exchanged or subjected to a maintenance work. When they are disassembled, they are usually disassembled completely into their components, as shown in FIG. 4. The pump head 21 is removed first, the seal holder 22 is removed next, followed by the head holder 25, and the return spring 28 and the plunger assembly 27 are finally removed. This sequence is reversed when the plunger assembly 27 is incorporated into the pump.
Although the structure of a pump for HPLC was described above in a simple form, real pumps are actually structured as shown in FIG. 5, having plunger assemblies 27 and return springs 28 in pairs, placed parallel to each other and undergoing their reciprocating motions alternately. When such a pump is disassembled, therefore, two return springs 28 must be removed, but this must be done by removing the screws of the head holder 25 while the return springs 28 are very cautiously stretched. Similarly, when the pump is assembled, the head holder 25 must be affixed inside the housing 26 by means of screws while the return springs 28 are compressed carefully. Such care must be taken when assembling and disassembling the pump because the return springs 28 may otherwise spring out to cause damage to the plunger assembly 27. In any event, it has been a precarious job to assemble and disassemble such a pump because of the existence of the return springs 28.